In recent years, from the standpoint of reduction of environmental burdens, there has been a need for a clean liquid fuel which has a low content of sulfur and aromatic hydrocarbons and is compatible with the environment. Thus, in the oil industry, a Fisher-Tropsch synthesis method (hereinafter abbreviated as “FT synthesis method) using carbon monoxide and hydrogen as raw materials has been investigated as a method of manufacturing a clean fuel. The FT synthesis method has high expectations since it can manufacture a liquid fuel base stock which has an abundance of paraffin and which does not contain sulfur, for example, a diesel fuel base stock. For example, Patent Document 1 discloses a fuel oil compatible with the environment.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-323626
A synthetic oil obtained by the FT synthesis method (hereinafter may be referred to as “FT synthetic oil”) has a broad carbon number distribution. From the FT synthetic oil, it is possible to obtain an FT naphtha fraction containing a number of hydrocarbons having a boiling point of, for example, 150° C. or less, an FT middle fraction containing a number of hydrocarbons having a boiling point of 150° C. to 360° C., and an FT wax fraction heavier than the FT middle fraction.
There is a concern that the FT middle fraction has insufficient low temperature-performance if the fraction is not processed because the FT middle fraction contains a great quantity of n-paraffins.
Furthermore, a substantial quantity of the FT wax fraction is simultaneously produced. Therefore, if such FT wax fraction can be converted to lighter products by way of hydrocracking the FT fraction, this will result in increased production of a diesel fuel.
Accordingly, the FT synthetic oil is fractionated into the FT middle fraction and the FT wax fraction, and the FT middle fraction is hydroisomerized to increase the iso-paraffin content in order to improve its low temperature performance.
On the other hand, the FT wax fraction is hydrocracked to convert the FT wax fraction to lighter products, thereby increasing the amount of the middle fraction. Accordingly, a sufficient quantity of a diesel fuel having sufficient performance can be obtained as the middle fraction from FT synthetic oil.